1. Field of the Invention
This invention relates to microfabrication of machines and machine components using photolithographic processes. More particularly, this invention relates to an improvement in the microfabrication of machines and machine parts using photolithographic processes and high energy X-rays wherein the improvement provides for increased adhesion of the resist to the substrate after exposure to high energy X-rays and subsequent development of the resist.
2. Description of Related Art
Microfabrication of engineered structures is becoming a major area of technology. These engineered structures or microengineered components as they are known are similar in shape and function to their full-sized counterparts, however, the dimensions of the microengineered components are measured in microns or tens of microns, as compared to, inches or meters for their full-sized counterparts. Examples of typical microengineered structures include; gears, pumps, rotors, and mechanical sensors. The microengineered components can be fabricated by cutting the selected building material with electron beams or lasers. Alternatively the microengineered components may be formed or cast by special processes that use a combination of photolithography and etching of silicon and thin films that are similar in many ways to semiconductor microelectronic device fabrication.
Recent developments in deep-etch lithography provide an alternative to previous microfabrication techniques. With the use of high energy X-rays (synchrotron radiation), together with an electroforming process, it is possible to fabricate microengineered components. Developed in Germany, this technique is called Lithographie, Galvanoformung, und Abformung (hereinafter "LIGA"), which is translated from German to mean synchrotron radiation lithography, galvanoforming, and plastic moulding. The process is related to the fabrication of semiconductor integrated circuits and requires similar tooling, in addition to a high-energy light source and electroplating equipment. Cavities and structural components with dimensions on the order of a millimeter can be formed with tolerances of a few microns and finishes of 100-2000 angstroms. Although synchrotron sources are sometimes available at nominal cost, obviously, the technique has high tooling costs, but after an initial investment, it allows large production quantities to be made, reproducibly and a relatively low incremental cost. The process is shown schematically in figures one to four. Secondary molds formed from the masters can be used to electroform (galvanoform) many copies of the structures, without the need of additional radiation.
"Resist" as used herein is a term of art for a coating layer for use in the manufacture of semiconductors, circuit boards, and the like. The resist is chemically modified upon exposure to radiation of a selected wavelength. A "mask" is used in combination with the resist permitting exposure of selected sections of the resist. Sections of the resist not exposed typically remain insoluble in a developer, while exposed sections become soluble in a developer. The reverse may also be the case depending on the resist/developer system. The exposed section in the present case is removed upon development. After the exposed sections of the resist have been removed, a number of chemical processes can be selectively undertaken on the underlying support.
Problems arise when the resist, typically, poly-methyl methacrylate (hereinafter "PMMA") is exposed, through a suitable mask, to high energy X-rays radiation. The high energy X-rays radiation passes through the unmasked resist or PMMA and enters the substrate. The substrate, upon exposure to the primary high energy X-rays radiation emits secondary electron emissions or photoemissions. The secondary emissions are not coherent and unidirectional like the primary high energy X-rays radiation. Secondary sources of radiation expose the shielded resist, and thereby chemically converting it in the same manner as the unmasked resist that is exposed to the primary high energy X-rays radiation. Exposures of the shielded resist occurs at the base of the resist adjacent to the resist/substrate interface. During the subsequent development of the resist, the resist exposed to the secondary emissions is removed in the same manner as the resist exposed to the primary high energy X-rays radiation.
The X-ray mask does not absorb all the radiation even in the opaque or "absorber" regions. X-rays of higher energy penetrate through the absorber portions and pass through the resist. As the resist does not absorb these higher energy X-rays significantly, it is not affected by them and the necessary contrast between the exposed and unexposed regions of the mask that permits selective dissolution is achieved. While these high energy X-rays pass through the resist without being absorbed, they are absorbed when they enter the substrate or the plating base which in typically metallic and has a higher absorption cross section at these X-ray energies than the resist. This results in the generation of secondary (auger) electrons and photoradiation at the interface of the resist and substrate in the regions under the absorber. This radiation penetrates into the resist and is absorbed at the interface within the first 3-5 micrometers. The depth to which the secondary radiation will penetrate depends on the energy of the primary synchrotron X-rays and increases with it. In addition radiation of similar origin which occurs in the exposed regions (under the clear or "carrier" portion of the mask) is non-directional and penetrated under the absorber regions. This render the interface regions of the resist under the absorber regions soluble as the regions under the carrier regions through similar chemical modifications.
This interfacial secondary radiation also causes another problem. It results in an excessive exposure of the resist under the carrier (clear) portion of the mask at the interface since the exposure there is the sum of the directly absorbed high energy X-rays and the secondary electron exposure from the interface. Over exposure of the resist results in the generation of large amounts of gases within it due to chemical modifications of the resist. These gases now begin to form bubbles in the resist at the substrate interface which results in the lifting off of the resist even before it is developed. If this problem is to be ordinarily avoided, a lower level of exposure will have to be used to avoid gas generation which will result in slower development of the resist and a reduction in contrast between the exposed and unexposed parts of the resist.
Both of these problems worsen when substrates and plating bases with large X-ray absorption cross sections are used. The desired device often does not permit arbitrary choice of material for substrate. Moreover, desirable plating bases that give good adhesion of the plated metal such as gold and substrates such as copper which is required for many devices have large absorption in the X-ray energies of relevance.
These problems do not arise when low level primary high energy X-rays sources in the range of 1 to 3 kV range are used to expose the resist since (a) the intensity of the secondary radiation is low and (b) the thickness of the secondary radiation exposed resist at the interface is extremely thin and it is difficult to dissolve this portion in the developer. However, as more powerfull high energy X-rays sources in the range of 10 to 30 kV become available and as the need arises to use these sources to fabricate thicker resists the problem will become more evident as the thickness of the secondary radiation exposed resist under the absorber becomes significant and the level of the exposure higher. One possible solution to the problem is to use thicker layers of material in the opaque (absorber) section of the photomask, however, when high energy X-rays are used the fabrication of masks with a sufficiently thick layer of gold (more than 50 micrometers) becomes difficult. Further, as the layer of opaque material is made thicker to block X-rays internal mechanical stresses increase within the mask resulting in mechanical failure of the mask.